<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>

        html {
            font-size: 10px;
        }

        body {
            font-size: 1em;
        }

        /*.box {*/
        /*    font-size: 1em;*/
        /*    width: 100px;*/
        /*    height: 100px;*/
        /*    background-color: red;*/
        /*    border: 1em solid #0066ff;*/
        /*}*/

        .box {
            font-size: 1em;
            width: 100px;
            height: 100px;
            background-color: red;
            border: 1rem solid #0066ff;
        }
    </style>
</head>
<body>
<div class="box"></div>
</body>
</html>
